Scanning tunnel-current-detecting device and method for detecting tunnel current and scanning tunnelling microscope and recording/reproducing device using thereof

ABSTRACT

A scanning tunnel-current-detecting device comprising at least two probe electrodes supported by a supporting member, a means for placing a sample in proximity to the probe electrodes, a means for applying voltage between the probe electrodes and the sample, at least one of the probe electrodes being provided with a mechanism for measuring and compensating variation of the distance between the supporting member and the sample, is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a scanning tunnel-current-detectingdevice comprising a mechanism for canceling variations caused by thermaldrifts and mechanical vibration, and a method for detecting a tunnelcurrent and to a scanning tunnelling microscope and arecording/reproducing device utilizing a method for detecting tunnelcurrent.

2. Related Background Art

Recently a scanning tunnelling microscope (hereinafter referred to asSTM) has been developed [G. Binnig et al., Helvetica Physica Acta, 55,726 (1982)] which enables direct observation of electronic structure ofthe atoms on the surface of a conductor, and allows a measurement of animage of real space of not only a single crystal but also an amorphousmaterial with high resolution. This measurement has an advantage that anobservation can be made with low electric power without impairing amedium by a current. Moreover, this measurement can be conducted in anatmospheric environment and is applicable to various materials, so thatthe method is promising in a variety of technical applications.

The STM utilizes a tunnel current which flows between a metallic probe(or a probe electrode) and an electroconductive material (or a sample)when an electric voltage is applied therebetween and the both arebrought into proximity as close as approximately 1 nm to each other.This current is extremely sensitive to the change of the distancebetween the probe electrode and the sample, so that the scanning with aprobe at a constant tunnel current allows depiction of the surfacestructure of the real space and simultaneously gives variousinformations regarding the whole electronic clouds of surface atoms.

To the STM for this purpose, a vibration isolator is indispensable whichreduces external disturbances caused by floor vibration, and a minutedeformation of constituting material caused by ambient temperaturevariation to less than a resolution limit.

Generally, for eliminating the influence of vibration, passive measuresare taken such as a method of reducing vibration by dissipating avibration energy with a damper element of a dynamic vibration isolator,and a method of lowering resonance frequency by employing a relativelymassive body as the supporter or the stand to increase a resistance tovibration.

Not so serious problem is encountered thereby in observation of a localatomic arrangement of a sample by locally scanning a probe electrode ofSTM. However, in observation of the surface state of a sample over arelatively large area by scanning a probe electrode of STM for a longtime, there arises a problem that an influence of temperature driftcaused by a thermal contraction or expansion of the member constitutingSTM and the sample to be measured become significant, and lowering themeasurement precision ensues. The variation caused by the temperaturedrift may sometimes reach approximately 0.5 μm (in the Z direction).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a scanningtunnel-current-detecting device and a method for detecting the tunnelcurrent which are highly precise and free from the disadvantages of theprior art and from the influence of a temperature drift caused by longtime of probe electrode scanning.

Another object of the present invention is to provide a scanningtunnelling microscope and a recording/reproducing device which causelittle measurement error even when it is used for long time ofmeasurement, and a method for detecting a tunnel current.

The above objects are achieved by the present invention.

According to an aspect of the present invention, there is provided ascanning tunnel-current-detecting device comprising at least two probeelectrodes supported by a supporting member, a means for placing asample in proximity to the probe electrode, a means for applying voltagebetween the probe electrode and the sample, at least one of the probeelectrode being provided with a mechanism for measuring and compensatingvariation of the distance between the supporting member and the sample.

According to an another aspect of the present invention, there isprovided a scanning tunnelling microscope comprising at least two probeelectrodes supported by a supporting member, a means for placing asample in proximity to the probe electrode, a means for applying voltagebetween the probe electrode and the sample, at least one of the probeelectrode being provided with a mechanism for measuring and compensatingvariation of the distance between the supporting member and the sample.

According to a further aspect of the present invention, there isprovided a method for detecting a tunnel current comprising employing atleast two probe electrodes supported by a supporting member, and stepsof bringing a sample in confrontation with and in proximity to the probeelectrodes such that tunnel current flows, and measuring andcompensating variation of the distance between the supporting member andthe sample.

According to an another further aspect of the present invention, thereis provided a recording/reproducing device comprising at least two probeelectrodes supported by a supporting member, a means for placing arecording medium in proximity to the probe electrodes, a means forapplying voltage between the probe electrodes and the recording medium,at least one of the probe electrodes being provided with a mechanism formeasuring and compensating variation of the distance between thesupporting member and the recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an STM provided with a mechanismfor compensating variation caused by thermal drifts and mechanicalvibrations.

FIG. 2 is a perspective view of the STM with emphasis of the sample forexplaining Example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a scanning tunnel-current-detectingdevice comprising a second probe electrode (a position-controlling tip)provided separately from a first probe electrode for observation, and avariation-compensating mechanism constituted of a feedback servo circuitwhich detects a variation of the member constituting STM and the sampleto be measured caused by a temperature drift from the change of a tunnelcurrent flowing between the second probe electrode and anelectroconductive sample, and which drives a fine variation-compensatingdevice in accordance with the signal of detection.

The Examples below describe the constitution of the present invention indetail.

EXAMPLE 1

FIG. 1 shows a block diagram of an STM comprising a variationcompensating mechanism of a preferred embodiment of the presentinvention.

The whole device is mounted on vibration-damping member 101 having arelatively large mass so that a vibration of high frequency fromexternal disturbance can be intercepted.

The relative positions of position-controlling tip 109 and observationtip 111 to electroconductive sample 108 can be chosen as desired bymeans of XY stage 102 within a plane (XY) direction and coarse adjustingmechanism 103 within a height (Z) direction. The numeral 104 donotes adriving circuit for driving coarse adjusting mechanism 103. Observationtip 111 is capable of scanning within a plane direction with a constanttunnel current maintained by cylindrical observation piezo-element 112,where the positional variation of cylindrical observation piezo-element112 in the Z direction corresponds to the surface state ofelectroconductive sample 108. Current amplifier 114 amplifies the tunnelcurrent flowing between observation tip 111 and electroconductive sample108. Servo circuit 115 serves to drive cylindrical observationpiezo-element 112 in the Z direction so as to keep the tunnel currentconstant when observation tip 111 scans.

Cylindrical observation piezo-element 112 for driving observation tip111 is integrated through piezo-element supporting member 116 withposition-controlling piezo-element 110 for driving position-controllingtip 109 as shown in FIG. 1.

While the surface is observed by scanning the surface ofelectroconductive sample 108 with observation tip 111, the tunnelcurrent is measured at a specified position of electroconductive sample108 by position-controlling tip 109.

This tunnel current will vary on receiving an external disturbance suchas vibration. Accordingly, variations caused by thermal drifts,vibrations, etc. can be cancelled mechanically by driving finevariation-compensating mechanism 105 in the Z direction by movingelectroconductive sample 108 through position-controlling servo circuit106.

Different from the above-mentioned mechanical correction, othercorrection methods also are feasible in which the variation read byposition-controlling tip 109 is subtracted from the observed value readby observation tip 111 on the basis of data analysis with microcomputor117.

These methods enable observation without an influence of minutedeformation of the construction material caused by an external vibrationor an environmental change when observation tip 111 scans on the sample.

Incidentally, position-controlling piezo element 110 is employed forsetting preliminarily the suitable tunnel current value for controllingthermal drifts, etc. by deciding the position of position-controllingtip 109 in the Z direction before beginning the surface observation, andthe preliminarily set driving voltage is kept constant during theobservation.

The numeral 113 is a circuit for driving position-controlling piezoelement 110, and the numeral 107 is a current amplifier for amplifying atunnel current flowing between tip 109 and sample 108.

The devices described above are respectively controlled by centralmicrocomputor 117. And the numeral 118 denotes a display instrument.

The tunnel current at a specified position was measured by observationtip 111, ten times with the feed back system of the present inventionemployed against thermal drift and vibration, etc.; and ten timeswithout employing the above feedback system for comparison. Theoperation of the feed back system was found to reduce the variationcaused by the temperature drift to 1/100 or less of the case withoutemploying the feedback system, by which the effect of the presentinvention was confirmed.

EXAMPLE 2

Another example is shown below. As shown in FIG. 2, use ofposition-controlling tip 109 and three sets of finevariation-compensating mechanism 105 for leveling the sample surfacebefore the observation of the electroconductive sample enabled themaintenance of the leveling during the observation.

A graphite surface was observed with this device to obtain satisfactorydata.

EXAMPLE 3

The device of the present invention as used for a recording/reproductingapparatus is shown below.

The constitution of the recording/reproducing apparatus is basicallysimilar to the block diagram shown in FIG. 1. A recording medium, havingprovided with a recording layer partially on the graphite, was used assample 108. Such the recording medium was so fabricated that therecording layer was positioned under probe electrode 111 and the surfaceof graphite was positioned under probe 109.

An LB layer (one layer) of squarilium-bis-6-octylazulene was used as arecording layer. The LB layer was made according to a method of JPLaid-Open No. 63-161,552.

Recording/reproducing was carried out as follows: The probe voltage of1.0 V was applied between probe electrode 109 and the graphite, and thedistance (Z) between probe electrode 109 and the graphite surface was soadjusted that the probe current (Ip) at a specified position was made10⁻⁹ A by means of fine variation-compensating mechanism 105.

Then, (+) was applied on probe electrode 111, (-) was applied on thegraphite, and a rectangular pulse voltage more than threshold voltageV_(th).ON to change a recording layer to a low resistance conditions (ONconditions) was applied to cause the ON conditions. Keeping the distance(Z) between probe electrode 111 and the graphite, a probe voltage 1.0 Vwas applied between probe electrode 111 and the graphite, and probecurrent (Ip) was measured. It was confirmed to be the ON conditions bydetecting a current of about 0.5 mA.

Upon setting the probe voltage of 10 V, which was more than thresholdvoltage V_(th).OFF, to change a recording layer from the ON to the OFFconditions and applying it again at the recording position, therecording conditions were erased and it was confirmed to be transferredto the OFF conditions.

In carrying out the recording/reproducing, variations caused by thermaldrifts, vibrations, etc. could be cancelled by driving finevariation-compensating mechanism 105 by means of position-controllingservo circuit 106, keeping the tunnel current constant, by measuring thetunnel current flowing between probe electrode 109 and the graphitesurface.

As described above, the STM provided with a mechanism for removingvariation caused by thermal drifts and mechanical vibrations can beexempted, in observation, from the influence of vibration of an angstromorder and a minute deformation of the construction material caused bytemperature change.

The device of the present invention may preferably used for a tunnellingcurrent detecting device other than a tunnel microscope, such as arecording-reproducing apparatus and the like.

We claim:
 1. An information detecting device for detecting aninformation of a sample by means of a probe electrode in proximity to asample surface, comprising at least a first probe electrode and a secondprobe electrode, which can be driven independently, supported by asupporting member, and provided with a driving means for driving saidfirst probe electrode, and a feedback system for compensating adeviation of a distance between said first probe electrode and saidsample surface by adjusting a distance between said sample surface andsaid supporting member so as to keep substantially constant a currentflowing between said second probe electrode and said sample surface. 2.A scanning tunneling microscope for observing a sample surface by meansof a probe electrode in proximity with said sample surface, comprisingat least a first probe electrode and a second probe electrode, which canbe driven independently, supported by a supporting member, and providedwith a driving means for driving said first probe electrode, and afeedback system for compensating a deviation of a distance between saidfirst probe electrode and said sample surface by adjusting a distancebetween said sample surface and said supporting member so as to keepsubstantially constant a current flowing between said second probeelectrode and said sample surface.
 3. A method for detecting aninformation of a sample by means of a probe electrode in proximity to asample surface, comprising using at least a first probe electrode and asecond probe electrode, which can be driven independently, supported bya supporting member, and detecting said information of the sample bymeans of said first probe electrode, compensating a deviation of adistance between the first probe electrode and said sample surface byadjusting a distance between said sample surface and said supportingmember so as to keep substantially constant a current flowing betweensaid second probe electrode and said sample surface.
 4. Arecording/reproducing device for recording an information into arecording medium and/or for reproducing or erasing a recordedinformation by means of a probe electrode in proximity to a recordingmedium, comprising at least a first probe electrode and a second probeelectrode, which can be driven independently, supported by a supportingmember, and provided with a driving means for driving said first probeelectrode, and a feedback system for compensating a deviation of adistance between said first probe electrode and said recording medium byadjusting a distance between said recording medium surface and saidsupporting member so as to keep substantially constant a current flowingbetween said second probe electrode and said recording medium.
 5. Amethod for adjusting a distance between a probe electrode and a samplesurface in a method for detecting an information of sample by means of aprobe electrode in proximity to a sample surface, comprising using afirst probe electrode and a second probe electrode, which can be drivenindependently, supported by a supporting member, and compensating adeviation of a distance between said first probe electrode and saidsample surface by adjusting a distance between said sample surface andsaid supporting member so as to keep substantially constant a currentflowing between said second probe electrode and said sample surface. 6.The method for adjusting a distance between a probe electrode and asample surface according to claim 5, wherein said first probe electrodeis connected with said supporting member through said driving means fordriving said first probe electrode.
 7. The method for adjusting adistance between a probe electrode and a sample surface according toclaim 5, wherein said driving means is a piezo element.
 8. The methodfor adjusting a distance between a probe electrode and a sample surfaceaccording to claim 5, wherein said second probe electrode is kept at aspecified position above said sample surface.
 9. A method for keeping asample surface horizontal with respect to a supporting member in amethod for detecting sample information by means of a probe electrodesupported by a supporting member in proximity to a sample surface,comprising using at least three probe electrodes on said supportingmember capable of being driven independently, and adjusting a positionalrelationship between said sample surface and said supporting member tokeep substantially constant current flowing between each of said atleast three probe electrodes and said sample surface, respectively. 10.The method for keeping a sample surface horizontal to a supportingmember according to claim 9, wherein said probe electrodes are kept atspecified position above said sample surface.
 11. A method for detectingan information of sample by means of a probe electrode in proximity to asample surface, comprising using at least four probe electrodes, whichcan be driven independently, supported by a supporting member, anddetecting said information of sample by means of the fourth probeelectrode, while adjusting a distance between said sample surface andsaid supporting member so as to keep substantially constant each currentflowing between said first to third probe electrodes and said samplesurface, respectively.
 12. The method for detecting an informationaccording to claim 11, wherein said sample surface and said supportingmember are kept horizontally, by adjusting a distance between saidsample surface and said supporting member.
 13. The method for detectingan information according to claim 11, wherein said first to third probeelectrodes are kept at specified positions above said sample surface.14. The method for detecting an information according to claim 11,wherein said fourth probe electrode is connected with said supportingmember through a driving means for driving said fourth probe electrode.15. A scanning tunneling microscope for observing a sample surface bymeans of a probe electrode in proximity to a sample surface, comprisingat least four probe electrodes, which can be driven independently,supported by a supporting member, and provided with a means foradjusting a positional relationship between the sample surface and saidsupporting member to keep substantially constant current flowing betweeneach of said first to third probe electrodes and said sample surface,respectively, to maintain said sample surface substantially horizontalwith respect to the supporting member, and a driving means for drivingsaid fourth probe electrode.
 16. The scanning tunneling microscopeaccording to claim 15, wherein said fourth probe electrode is connectedwith said supporting member through a driving means for driving saidfourth probe electrode.
 17. The scanning tunneling microscope accordingto claim 15, wherein said driving means is a piezo element.
 18. Thescanning tunneling microscope according to claim 15, wherein said firstto third probe electrodes are kept at specified positions above saidrecording medium.
 19. An information detecting device for detecting aninformation of a sample by means of a probe electrode in proximity to asample surface, comprising a first probe electrode and at least oneother probe electrode, which can be driven independently, a drivingmeans for driving said first probe electrode, and a feedback system forcompensating for deviation in a distance between said first probeelectrode and said sample surface by adjusting the position of thesample so as to keep substantially constant a current flowing betweenthe at least one other probe electrode and said sample surface.